The present disclosure relates to a method for treating a substrate with a polymer film-forming composition. More particularly, the disclosure relates to a paper or paperboard manufacturing method comprising the steps of applying a polymer film-forming coating to a substrate, and, bringing the polymer coating into contact with a heated surface while the polymer coating is still in a wet state. The resulting polymer layer has a smooth surface with voids (e.g., bubbles) just below the surface. In certain embodiments, the polymer coating may comprise a crosslinkable hydrogel, and a crosslinking solution may be applied to the polymer coating on the substrate surface thereby forming at least a partially crosslinked polymer coating then placed into contact with a heated surface. The present disclosure also relates to a treated substrate product. The present disclosure also relates to a method for treating a substrate with a polymer film-forming composition, and bringing the substrate into contact with a heated surface in a pressure nip.
Paper is manufactured by an essentially continuous production process wherein a dilute aqueous slurry of cellulosic fiber flows into the wet end of a paper machine and a consolidated dried web of indefinite length emerges continuously from the paper machine dry end. The wet end of the paper machine comprises one or more headboxes, a drainage section and a press section. The dry end of a modern paper machine comprises a multiplicity of steam heated, rotating shell cylinders distributed along a serpentine web traveling route under a heat confining hood structure. Although there are numerous design variations for each of these paper machine sections, the commercially most important of the variants is the fourdrinier machine wherein the headbox discharges a wide jet of the slurry onto a moving screen of extremely fine mesh.
The screen is constructed and driven as an endless belt carried over a plurality of support rolls or foils. A pressure differential across the screen from the side in contact with the slurry to the opposite side draws water from the slurry through the screen while that section of the screen travels along a table portion of the screen route circuit. As slurry dilution water is extracted, the fibrous constituency of the slurry accumulates on the screen surface as a wet but substantially consolidated mat. Upon arrival at the end of the screen circuit table length, the mat has accumulated sufficient mass and tensile strength to carry a short physical gap between the screen and the first press roll. This first press roll carries the mat into a first press nip wherein the major volume of water remaining in the mat is removed by roll nip squeezing. One or more additional press nips may follow.
From the press section, the mat continuum, now generally characterized as a web, enters the dryer section of the paper machine to have the remaining water removed thermodynamically.
Generally speaking, the most important fibers for the manufacture of paper are obtained from softwood and hardwood tree species. However, fibers obtained from straw or bagasse have been utilized in certain cases. Both chemical and mechanical defiberizing processes, well known to the prior art, are used to separate papermaking fiber from the composition of natural growth. Papermaking fiber obtained by chemical defiberizing processes and methods is generally called chemical pulp whereas papermaking fiber derived from mechanical defiberizing methods may be called groundwood pulp or mechanical pulp. There also are combined defiberizing processes such as semichemical, thermochemical or thermomechanical. Any of the tree species may be defiberized by either chemical or mechanical methods. However, some species and defiberizing processes are better economic or functional matches than others.
An important difference between chemical and mechanical pulp is that mechanical pulp may be passed directly from the defiberizing stage to the paper machine. Chemical pulp on the other hand must be mechanically defiberized, washed and screened, at a minimum, after chemical digestion. Usually, chemical pulp is also mechanically refined after screening and prior to the paper machine. Additionally, the average fiber length of mechanical pulp is, as a rule, shorter than that of chemical pulp. However, fiber length is also highly dependent upon the wood species from which the fiber originates. Softwood fiber is generally about three times longer than hardwood fiber.
The ultimate properties of a particular paper are determined in large part by the species of raw material used and the manner in which the paper machine and web forming process treat these raw materials. Important operative factors in the mechanism of forming the paper web are the headbox and screen.
Coated paper or paperboard used for printing and for packaging is generally required to have high level of gloss, excellent smoothness, and excellent printability, as well as certain strength and stiffness characteristics.
If the coated paper or paperboard has a high stiffness, it can pass smoothly through high-speed printing or packaging machines with less feeding jams. Higher stiffness paper can be advantageously used in books, magazines, and catalogues, because it provides a feel of hardness or heaviness similar to a hardcover book. For packaging, high stiffness is necessary for maintaining the structural integrity of the paperboard product during filling and in subsequent use.
Stiffness has close relationship to the basis weight and density of paper. There is a general trend that stiffness increases as the basis weight increases (for a given caliper), and decreases as the paper density increases (for a given basis weight). Stiffness and other properties can be improved by increasing basis weight. However, this would result in a product utilizing more fibers, which add cost and weight. Therefore, coated paper or paperboard with high stiffness but moderate basis weight is desirable. Paper with moderate basis weight is also more economical because less raw material (fiber) is utilized. In addition, shipping costs based on weight are less for low basis weight paper.
In addition to high stiffness, coated paper or paperboard which must be printed is often required to have high gloss and smoothness. For coated paper or paperboard to have such quality characteristics, density typically must be increased to some extent to allow for a usable printing surface. Smoothness is normally achieved by calendering. However, calendering will cause a reduction in caliper, which typically results in a corresponding reduction in stiffness. The calendering process deteriorates the stiffness of paper by significantly reducing caliper and increasing the density. The base sheet for conventional coated board grades typically is heavily densified by calendering to provide a surface roughness low enough to produce final coated smoothness acceptable to the industry. These calendering processes, including wet stack treatment, may increase density by as much as 20% to 25%.
Thus, the relationship between gloss and stiffness and between smoothness and stiffness are generally inversely proportional to each other, for a given amount of fiber per unit area. Packaging grades are sold based on caliper, so manufacturing processes that reduce the caliper (increasing the density of the board) decrease the selling price. Processes that cause less caliper reduction save material costs. Caliper is measured in “points”, where a point=0.001 inches. For example, the conventional method for making a 10-point board requires the use of a board having a thickness of greater than 12 points prior to calendering. It would be desirable to be able to produce a finished board having approximately the same thickness as the starting substrate.
Improvements in the calendering process including moisture gradient calendering, hot calendering, soft calendering, and belt calendering slightly improved stiffness for a given caliper but did not change the fundamental ratio between caliper, stiffness, smoothness, and printing properties.
Various proposals have been made to improve the stiffness of coated paper or paperboard without calendering for printing. For example, several proposals include high softwood content in the raw stock, addition of specially engineered fibers in the raw stock, addition of highly branched polymers within the raw stock, and high amounts of starch or copolymer latex with a high glass transition temperature (commonly referred to as “Tg”) within the coating formulation.
However, potential drawbacks to these methods of stiffness improvement are that although they are useful in improving paper stiffness, they could potentially degrade the smoothness, gloss, and/or printability of the coated paper obtained.
For the reasons mentioned above, it has been very difficult to obtain satisfactory paper smoothness without increasing density. Other methods can be used for changing the density/smoothness relationship in paper and paperboard grades. Applying a paper coating is a very common way to enhance the surface properties of paper without causing the drastic increases in paper density typically associated with the levels of calendering required to obtain a certain level of smoothness. Preferably, the final coated surface should be uniform to provide acceptable appearance and printing properties.
Therefore, it would be desirable to provide a paper or paperboard product having the desired properties while maintaining the initial density of the sheet or minimizing the increase in density. Furthermore, it would be desirable to provide a paper or paperboard exhibiting improved smoothness without the concomitant increase in density associated with conventional methods for creating smoothness. Cast coating methods exist for producing a very smooth surface, but these methods are typically run at production rates slower than the speed of many paper machines.